About The Impedance Of Multimeters
Ever wondered why when you measure the voltage of a particular circuit with your digital and analogue meter, the result that you got were different? The reason for it usually is due to the impedance of the meter you are using. It is also depends on what kind of circuit you are measuring.
So what is impedance?
The definition of impedance is the measure of the opposition to electrical flow. It is measured in ohms and represented by the symbol Z.
Generally digital multimeter has a high impedance input of 1 Mega Ohm and greater and for analogue multimeter would be 20 Kilo Ohm for DC and 9 Kilo Ohm for AC.
So what does this Ohm value mean? If a multimeter that has high impedance input, it means when you are measuring the voltage of a point (or nodes) in an electronic circuit, very little current would goes through it (the multimeter) thus it would not disturb the circuit under test and you will get the voltage what you are expecting. If you are using a low impedance input multimeter such as the Analogue Multimeter, the volt meter will draw too much current from the circuit and will give you a false voltage reading. This is called as circuit loading (loading effect). In other words, your meter is an external load to the circuit you are measuring and will alter circuit performance.
For example, if you are measuring a point that has 3 volt using a high impedance input meter and you got approximately 3 volt, then you know that the point you are checking is good. For that same point if you use a low impedance input meter to check, you may get 2.90 or even 2.8 volt (depending on the type of circuit you are checking)! This means the low impedance input meter is disturbing the circuit you are measuring (loading effect) and gave you an inaccurate reading. If you do not know about the impact of multimeter impedance to a circuit, you may have thought that the circuit you are measuring have problem but the fact is that particular circuit you are checking is actually in working condition!
If this is the case then why do we still need to use a low impedance meter?
Now, don’t get me wrong that Analogue meter is of no use in checking the DC voltage due to its lower impedance (20 K ohm) as compare to digital multimeter (1 Mega ohm and above). If you just want to know if a DC voltage is present in an electronic circuit and the value is not that important to you, you can always use an Analogue meter to test it because it can give you a fast result by just looking at the pointer. For example, if you are checking a 5volt DC line of a power supply and your Analogue meter shows you 4.85 to 4.9 something volt, you know that this voltage would be acceptable because there would be tolerance in the output DC volt.
However, if you really want to know the precise voltage of the DC output then need to use a Digital multimeter to test it. The result could be between 4.98 to 5.01 volt depending on the accuracy of the digital multimeter. As for me when checking DC voltage of a circuit I would usually use a Digital Multimeter because it has a higher impedance input. Actually the 20K ohm impedance of Analogue multimeter would not severely impact the result you got as compare to the Multimeter that has even lower impedance value that I will briefly explain in the next paragraph.
Dual impedance meter
Nowadays you can find Digital multimeter that comes with dual impedance features. The first feature would be the high impedance input (usually 10 Megaohms) that built into the voltmeter range and the second feature (low impedance input usually between 2k to 5kohm depending on the brand of meter). Fluke named it as Auto-V/Lo Z and you need to deliberately set it to use this feature. Take a look at the Fluke 117 meter video below explaining about the Ghost voltage and how the low impedance range can help to solve it:
Another example would be testing a button cell or coin battery (or watch battery). If you test these kinds of batteries using a high impedance input meter, a weak battery may still show good 3 volt! However, once you have selected your meter to Lo Z test, the result would be different, you may get between 2 to 2.5 volt only because this Lo Z feature is sort of like putting a heavier load (2k to 5k ohm depending on the brand of meter) in which the weak battery can’t sustain the load draw thus the voltage will drop. Although UNI-T 191 Model has LoZ feature but that feature is only for the AC voltage and not DC voltage.
This is why before you buy any Digital multimeters you need to know the specification of the meter. In other words, not all Digital multimeter are designed/created to be the same in term of features and specification. Expensive and branded Digital multimeters has its own advantages over cheap Digital multimeters.
Conclusion- If you already have a digital multimeter with high impedance of 10 Mega Ohm, it is not necessarily you must get another meter that has the low impedance feature. It depends on your job application. Having a Digital and Analogue multimeter already can solve lots of electronics circuit problems. Of course with more specialized test equipment your troubleshooting job would be much easier because these test equipment can remove the guesswork and shortening your troubleshooting and repairing time. Hope you have learned something new today and if you have any questions, you can post a comment below that is related to this topic.
This article is brought to you by Jestine Yong. He is from Kuala Lumpur Malaysia and he loved electronics repair and blogging about electronics repair information. He is the author of the famous Basic Electronics Repair and SMPS Repair ebook . He is also a trainer and conduct electronics repair courses at Noahtech Electronics Training Center.
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